In a quadrature receiving device a signal is mixed down with versions of a local oscillator signal that are ninety degrees out of phase with each other. This results in an in-phase signal and a quadrature signal that represent the real and imaginary parts of a complex signal. The spectrum of the resulting complex signal corresponds to the spectrum of the original signal, frequency shifted by the local oscillator frequency. Unlike mixing with a single signal, this makes it possible to distinguish frequency components at frequencies above and below the local oscillator frequency.
One problem with this type of receiver is that it is vulnerable to what is called I/Q mismatch, which occurs when the two mixing operations are not carefully matched. Such I/Q mismatch results in spurious signal components, i.e. components in the mixed down signal that seemingly indicate components of the original signal that are not actually present in the original signal. Similar problems arise for transmission, when two low frequency signals are mixed up to form a high frequency signal. In this case, the high frequency signal may contain a spurious component due to I/Q mismatch.
Self calibration to eliminate I/Q mismatch is known from an article by Chia-Hu Tsu, titled “FPGA prototype for WLAN OFDM baseband with FTPE of I/Q mismatch self-calibration algorithm”. This article proposes to supply I/Q components of a single tone test signal low frequency signal in the baseband of a transmitter to mix these signals up to a high frequency signal. The power of the high-frequency signal is measured. This power varies at twice the frequency of the low frequency signal. The article proposes to use the phase of these variations to determine calibration parameters to eliminate I/Q mismatch. In addition to the single tone test signal, the article uses spectra of operational OFDM modulation signals to fine tune calibration.
This type of self tuning is quite complex.